Constantius is an important, but almost forgotten, figure. He came to the fore in or around 410 when he was appointed Magister Militum (Master of Troops) to Honorius, the young Emperor of the Western Roman Empire. His predecessor, Stilicho, had been murdered by his own troops and much of Gaul and Hispania had been overrun by barbarians or usurpers.

One by one Constantius eliminated the usurpers and defeated or came to terms with the various invading groups. Most notoriously, he allowed the Visigoths to settle in Gaul in return for their help in defeating the Vandals and Alans who had seized parts of Hispania, a decision with far-reaching consequences.

Constantius married Honorius' sister and was eventually proclaimed his co-emperor. However, the Eastern Roman Emperor, Honorius' nephew, refused to accept his appointment and Constantius was preparing a military expedition to enforce this recognition when he died suddenly, having been emperor for just seven months. Ian Hughes considers his career, assessing his actions in the context of the difficult situation he inherited.